The present invention relates to a method of shifting a powershift transmission.
Conventional powershift transmissions use solenoid controlled valves to control pressure to each clutch, and such transmissions change ratios by disengaging one or more clutches while simultaneously engaging one or more clutches. Such transmissions also rely on a signal that is representative of engine load to determine the pressure applied to the on-coming clutches
The shift smoothness is normally controlled by engaging the on-coming clutches at low pressure. The amount of pressure necessary is dependent on the load being transmitted by the transmission to the drive wheels. If the engagement pressure is too low, the vehicle may lose speed during the shift. If the engagement pressure is too high, the shift will be very aggressive and harsh.
Various problems can occur with this conventional type of system. For example, sometimes the on-coming clutch piston will not move enough to start engagement, even though the off-going clutch has been disengaged. Under load this will cause the vehicle to lose speed during the shift. Even though the solenoid valve is opened enough to provide the correct pressure, the on-coming clutch may not be engaged and cannot transmit torque.
Another problem is that the engine load signal may be misleading. For example, in agricultural tractor applications, there are conditions where much of the engine load may be used to power a hydraulic pump or power-take-off (PTO) implements. This may cause a harsh shift quality because the on-coming clutch pressure is commanded at high pressure when instead it should have been commanded low because actually only a small amount of the engine power was being transmitted to the drive wheels.
A method of reducing the problem of the on-coming clutch not getting filled is described in U.S. Pat. No. 5,580,332 issued in 1996 to Mitchell et al. In this method, the actual time to fill the clutch is determined during each shift. Depending on whether the clutch filled sooner than expected or later than expected, the fill time that is used for the next clutch engagement may be adjusted. The fill time is determined by bringing the off-going clutch down to a low stable value, while the on-coming clutch is brought up to fill pressure, then looking for the point in time where the off-going clutch slips or the torque converter speed ratio changes.
A method of controlling the shifting of powershift transmissions and overcoming both deficiencies is described in U.S. Pat. No. 6,193,630 and U.S. Pat. No. 6,435,049, both assigned to the assignee of this application. In this method, the first step in a shift is to bring an off-going clutch down in pressure until slip is detected. Slip is caused purely by the load on the vehicle, not by the on-coming clutch coming up in pressure. The off-going clutch is then maintained in a condition of slight slip while on-coming clutches are filled and swapping of other intermediate clutches are performed. Finally, the last swap is made between this off-going clutch and the final on-coming clutch.
With this method, under some conditions where there is little load on the drivetrain, it may take a considerable amount of time to detect slip, thus causing a delay in time from when the operator commands a shift by moving a shift lever and until the shift actually occurs. Such delays are disliked by operators because it gives them the impression of not being in control. Also with this method, only after slip is detected in the off-going clutch, is the on-coming clutch brought up to pressure. This further delays the completion of the actual shift.
Another disadvantage of this method is that the first detection of slip always results in the vehicle slowing down. During an upshift, it is not desirable to feel the vehicle first slow down before it finally speeds up.